The invention relates generally to resource allocation in wireless spread spectrum time division duplex communication systems using code division multiple access. More specifically, the invention relates to assigning time slots and codes in such systems.
FIG. 1 depicts a wireless spread spectrum time division duplex (TDD) communication system using code division multiple access (CDMA). The system has a plurality of base stations 201–207. Each base station 201, communicates with user equipments (UEs) 221–223 in its operating area or cell. Communications transmitted from the base station 201 to the UE 221 are referred to as downlink communications and communications transmitted from the UE 221 to the base station 201 are referred to as uplink communications. In addition to communicating over different frequency spectrums, spread spectrum TDD/CDMA systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective codes.
Since a signal sent using a particular code is distinguishable from other signals in the same spectrum, each code creates a virtual communication channel within the spectrum. For use in distinguishing signals originating from different cells, each base station 201–207 has an assigned scrambling code, c*scramb. To transmit a specific data signal in such a system, the data signal is mixed with its base station's scrambling code, c*scramb, and spread using its assigned channel code.
Also, to more efficiently use the spread spectrum, TDD/CDMA systems use repeating frames 26 divided into a number of time slots 241–24n, such as fifteen time slots, as illustrated in FIG. 2. In such systems, a communication is sent in selected time slots 241–24n using selected codes. Accordingly, one frame 26 is capable of carrying multiple communications distinguished by both time slot 241–24n and code. The use of a single code in a single time slot at a single frequency spectrum with a spreading factor of sixteen is referred to as a resource unit. If a lower spreading factor is used, the use of a single code in a time slot is considered more than a single resource unit. To illustrate, using a spreading factor of one for a code in a time slot is sixteen resource units.
A system using N time slots, S1–SN, M channel codes, Code 1-Code M, and O frequency spectrums, Frequency 1-Frequency O,is illustrated in the Matrix 28 of FIG. 3. Each empty box in the Matrix 28 represents a single resource unit (if a spreading factor of sixteen is used). This Matrix 28 has a total of M×N×O resource units. A typical TDD system uses 15 time slots, 16 channel codes and one or multiple frequency spectrums. Based on the bandwidth required to support a communication, one or multiple resource units are assigned to that communication.
One problem in such systems is assigning resource units in the presence of radio interference. Radio interference has multiple causes, such as nearby radio frequency sources and cross interference by signals transmitted in neighboring cells. Sending a communication over a resource unit with a high interference level may result in a loss of signal data.
One technique for dealing with this problem is to measure the interference level in each time slot immediately prior to assigning resource units to a communication. Only resource units in time slots having acceptable interference levels will be assigned to the communication. Although this technique reduces the possibility of signal data loss, it does not eliminate all resource units suffering unacceptable interference levels. Additionally, measuring the interference levels immediately prior to assignment requires extensive monitoring using valuable system resources. Accordingly, there exists a need for an alternative approach for assigning resource units.